We assess the impact of a post-grant review system in the U.S. patent system by comparing the “opposition careers” of EPO equivalents of litigated U.S. patents to those of a control group of EPO patents. We find that equivalents of litigated patent applications are more likely to receive patent protection than control group patents, and that the opposition rate for equivalents of litigated patents is about three times higher than for control group patents. Patents attacked under opposition are either revoked completely or narrowed in about 70 percent of all opposition cases. In the case of EPO equivalents of U.S. litigated patents, the appeal rate against opposition outcomes is considerably higher than for controlgroup patents. Based on our estimates, we calculate a range of net welfare benefits that would accrue from adopting a post-grant review system. Our results provide strong evidence that the United States would benefit substantially from adopting an administrative post-grant patent review.